Inhibitor to reduce mercury corrosion

ABSTRACT

Mercury corrosion of the interior of closed metal vessels, such as bulbs of temperature responsive devices, which contain mercury at elevated temperatures and pressures is reduced by disposing an inhibitor, such as a film of ferric oxide, within the interior of the vessel. The vessels which are normally exposed to an oxidizing atmosphere on their outer surfaces are constructed of a ferrous alloy containing an element such as chromium and/or nickel.

I Unlted States Patent 1191 Shopsky May 28, 1974 [54] INHIBITOR TO REDUCE MERCURY 2,415,309 2/1947 Stone 23/252 x CORROSON 2,442,223 5/1948 Uhlig 148/635 2,858,244 /1958 Long et al 148/635 X Inventor: Harvey J- p y, Latrobe, 3,218,861 11/1965 Moore et al... 73/37 [73] Assignee: Robertshaw Controls Company,

Richmond, Va. Przmary Exammer-R1chard C. Que1sser Assistant Examiner-Daniel M. Yasich [22] Filed: l' 1972 Attorney, Agent, or Firm-Anthony A. OBrien [21] Appl. No.: 247,772

' [57] ABSTRACT 52 us. c1. 73/3684, 29/195 M, 431/66 Mercury corrosion of the interior of closed metal 51 1111.01. c0111 5/02, 0231 15/00 8618, Such as bulbs of temperature responsive devices, 53] Field of Search 73/3684, 3363, 371; which contain mercury at elevated temperatures and 148/635; /133; 117/107; 23/252; pressures is reduced by disposing an inhibitor, such as 29/ M; 220/64; 431 /66 a film of ferric oxide, within the interior of the vessel. The vessels which are normally exposed to an oxidiz- [56] R f r n Cited ing atmosphere on their outer surfaces are constructed UNITED STATES PATENTS of a ferrous alloy containing an element such as chr0- 2,l75,77l 10/1939 Giles 117/107 X mlum and/or mckel' 2,366,091 12/1944 Eskin 73/3684 4 Claims, N0 Drawings 1 INHIBITOR TO REDUCE MERCURY CORROSION BACKGROUND OF THE INVENTION Field of the Invention mercury at high temperatures and high internal pressures.

Description of the Prior Art Mercury is utilized in a number of high temperaturehigh pressure applications, such as in power generation apparatus and in temperature control devices. Mercury has a number of desirable physical properties including a low melting point, high volumetric expansion, vapor pressure, flow ability and stability at high temperatures. In addition, mercury is readily available, low cost and relatively easy tohandle.

Unfortunately, however, mercury is difiicult to contain in metal vessels at high temperatures and pressure due to its ability to dissolve most common metals. It has a particular affinity for nickel and chromium which are elements required for oxidation resistance in steel alloys at service temperatures between 1000 and 2000F. The oxidation resistance and service temperatures at which these alloys may be used is generally proportional to the amount of nickel or chromium in the alloy. On the other hand, it has been found that the solubility of these two elements, i.e. nickel and chromium, is an exponential function of the temperature of the mercury. In addition, it has been found that the rate of corrosion or solubility of chromium and nickel in mercury is proportional to the percentage of these elements in the steel alloy.

The utilization of mercury in various high temperature-high pressure applications is shown in prior art patents. For example, US. Pat. No. 2,366,091 is concerned with the use of special steel alloys to avoid mercury corrosion. U.S. Pat. No. 2,640,313 deals with a mercury filled sensing device.

The treatment of metals to modify their properties is shown in a number of prior art patents. For example, U.S. Pat. No. 1,690,378 discloses forming a protective coating by applying black iron oxide to a metallic substrate. U.S. Pat. No. 2,175,771 relates to treating chromium-bearing metals to make them corrosion resistant by heating the metal sufficiently to change its crystalline structure. U.S. Pat. No. 2,442,223 discloses a method of improving corrosion resistance of chromium alloys which comprises heating an iron-nickelchromium alloy to form a chromium oxide (Cr O coating. U.S. Pat. No. 2,547,536 relates to a method for forming a surface which is easily wettable by mercury. The nickel-iron substrate is heated to form an oxide layer to which mercury will readily adhere.

SUMMARY OF THE INVENTION The present invention is embodied in an apparatus including a closed metal vessel, such as a bulb of a temperature responsive device, containing mercury wherein an inhibitor, such as a film of ferric oxide, is disposed within the interior of the vessel, whereby mercury corrosion is minimized at high temperatures and high internal pressures.

A primary object of the present invention is to reduce mercury corrosion of closed metal vessels at high temperatures and pressures.

A further object of the present invention is to increase the service and material life of closed metal vessels containing mercury at high temperatures and pressures.

A still further object of the present invention is to increase the permissible operating temperature of closed metal vessels containing mercury at high temperatures and pressures.

Other objects and advantages of the present invention will become apparent from the following description.

DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention is advantageously used in temperature responsive devices which conventionally include a temperature sensing bulb containing mercury which is connected in fluid communication with a capillary tube which in turn is connected in fluid communication to an actuating element. These devices are well known, for example, in the art of temperature responsive controls which are suitable for high temperature use such as in the detection of flame at a burner.

The temperature sensing bulb of these temperature responsive devices should be constructed of a ferrous or steel alloy which is heat resistant and also oxidation resistant, since it will be exposed to a high temperature flame in an oxidizing atmosphere. Thus, the bulb should be capable of operating satisfactorily at elevated temperatures in the range of about 1000 to 2000F and at high internal pressures of the magnitude experienced when mercury is heated to these temperatures in a closed vessel. The ferrous or steel alloy for the bulb should contain a minor proportion of chromium, nickel and/or other elements, or combinations thereof, which increase the oxidation resistance of the alloy at high temperatures. Such alloys are well known in the art. Chromium stainless steel is the preferred construction material for the temperature sensing bulb. Chromium stainless steel develops its oxidation resistance by forming a stable chrome-oxide film on its, surface that increases to a degree in depth and corrosion resistance with age. High temperature scaling, atom of oxidation, is controlled 'by the amount of chromium in the alloy. Chromium is a powerful factor in improving resistance to scaling. Maximum service temperature in an oxidizing atmosphere is approximately proportional to the chromium content.

In accordance with the present invention, a film of an inhibitor is deposited on the inner surface of the temperature sensing bulb. The preferred inhibitor is ferric oxide. The inhibitor which may be in the form of a paste or liquid composition is applied to the interior of the bulb by coating, brushing, dipping or the like and then dried. Generally, a film of several mils thickness will be satisfactory, although thicker or thinner films may also be employed.

Without intending to be bound by theory, the present mechanism by which the inhibitors of the present invention operate involves the following considerations. Mercury, which behaves like a noble metal, is not attacked by oxygen -at ordinary temperatures. However, at approximately 350C mercury combines with-oxygen to form mercuric oxide which decomposes when heated to a higher temperature. It has been found that the attack on chromium or nickel ferrous alloys by mercury at elevated temperatures appears to be largely governed by a solution mechanism. It has further been found that the liquid boundary layer at the vessel wall attains saturation equilibrium of the solubles from a wetted wall surface in a relatively short time. it, therefore, appears that corrosion could be reduced or eliminated if the wall of the vessel could be kept passivated.

Since mercury has substantially no affinity for oxygen at temperatures exceeding 1000F, it appears that it is possible to increase or maintain the stable chrome oxide on the wall of the vessel by depositing ferric oxide inside the vessel in the area where the mercury corrosion is most likely to occur. Test work shows that ferric oxide when used as an inhibitor in the manner previously described, virtually eliminates mercury corrosion in burner applications. The exact mechanism of inhibition has not been proven, but it appears that the ferric oxide dissociates due to the temperature involved, and provides oxygen to maintain the stable chrome-oxide film'on the wall surface. It is thus theorized that this protective film prevents the mercury from wetting the wall surface which would make it active to mercury corrosion. On the basis of this mechanism of inhibition, the oxide of any element, whereby oxygen has a greater affinity for chromium than that element, should serve as an inhibitor providing the element is compatible with the mercury for the intended application.

It has been found that the present invention reduces mercury corrosion to a minimum in an application such as temperature control of a gas or oil burner. This can be accomplished while using common steel materials for construction of the devices of the present invention. Also, the service and material life of the devices is substantially increased. The present invention also permits the temperature responsive devices to be utilized at a higher temperature than conventional devices without sacrificing service life due to increased mercury corrosion.

The ferric oxide coating may be deposited or coated on the inside surface of the bulb. However, alternately the ferric oxide could be disposed in the interior of the bulb in the form of a slug. Another alternative would be to introduce an iron slug having a ferric oxide film into the interior of the bulb prior to fusion welding. In a still further alternative, a thin iron bushing is slipped into the bulb before the bulb end is swaged. The bulb would then be swaged and cleaned in the usual manner after which the bulb would be placed in an atmosphere which would induce a ferric oxide film to form on the iron bushing. Fusion welding and charging would then follow in the usual manner.

In accordance with the present invention a temperature responsive device was constructed by depositing a coating of ferric oxide on the inside surface of bulb made of 446 stainless steel. This was accomplished by making a slurry of fine powdered ferric oxide in water and injecting this into the swaged end of the bulb before fusion welding. The water was baked off the slurry leaving a cake of ferric oxide on the inside wall that was not sucked out during bulb evacuation for charging. The device of the present invention containing the ferric oxide coating was then tested in comparison with a conventional temperature responsive device without a coating wherein the bulb was made of 446 stainless steel. The tests have shown to date that whereas the conventional device failed under test conditions after about 12 months, the device of the present invention exhibited essentially no corrosion after more than 18 months on test.

Inasmuch as the present invention is subject to many modifications, variations and changes in detail, it is intended that all matter contained in the foregoing description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A temperature responsive device capable of withstanding an elevated temperature in a range of about l000 to 2000F. with a minimum of mercury corrosion comprising an evacuated temperature sensing bulb for use in a temperature responsive means, said bulb containing mercury as a fluid responsive to temperature changes, said bulb being constructed of a heatresistant ferrous alloy containing sufficient chromium to render the exterior surface of the bulb oxidation-resistant at said elevated temperature, and

claim 1 wherein said metal oxide is an iron oxide.

' i i =l i 

2. A temperature responsive device as defined in claim 1 wherein said metal oxide is a film of ferric oxide deposited on the interior surface of said bulb.
 3. A temperature responsive device as defined in claim 1 wherein said heat-resistant ferrous alloy contains nickel.
 4. A temperature responsive device as defined in claim 1 wherein said metal oxide is an iron oxide. 